We all know that kids are susceptible to advertising, so when fast food chains started using popular cartoon characters to promote their high-calorie fare, it seemed like a no-win situation for parents hoping to teach their tykes about healthy eating.

So in 2010, a California county became the first in the nation to ban toys from fast-food children’s meals that were high in calories, salt, fat and sugar, with the idea being that eliminating the toys would make the meals less enticing to kids. And researchers say that the ordinance seems to be having an effect — at least on the way the fast-food chains advertise their goods.

In a preliminary look at whether the ban actually improved the menu options offered and marketed by fast-food chains, Jennifer Otten, a research fellow at Stanford University’s Prevention Research Center, and her colleagues included eight restaurants — four were in regions in Santa Clara county that had to comply with the ban, while the other four were in the same restaurant chains outside the ban’s jurisdiction.

Otten and her group scored each restaurant on what menu items were available, whether whole grain options were offered, and what proportion of the food met the Dietary Guidelines for Americans (DGA) criteria for healthy eating. The researchers also looked at restaurants’ marketing practices, such as whether toys were still available with children’s meals, as well as the calorie, fat, salt and sugar contents of these meals.

In the four months after the ban took effect, Otten found, the restaurants that adhered to the ban showed a threefold improvement in scores, while those that didn’t have to comply with the ban showed minimal changes. The Santa Clara ban was specifically focused on disassociating toys with foods offered at fast-food chains, so the restaurants that complied did everything from eliminating the toys altogether to offering them only with meals that met DGA criteria. In the latter case, parents could purchase the toys, but they had to ask for and buy them separately.

One restaurant also removed the posters placed under the cashiers — at a kid-friendly level — advertising toys with meals. Others changed their menu displays to include pictures of only those children’s meals that met DGA requirements. “They provided better on-site nutritional guidance, and did a better job of promoting and providing healthier meals,” says Otten of the restaurants responding to the ban.

However, the study found that the restaurants didn’t actually increase the number of healthy options available. Before the ban, only 4% of the children’s meals met DGA standards — that number that didn’t change in the four months after the law took effect.

The current study also did not assess whether consumers changed their fast-food choices based on the restaurants’ changes, but previous studies on the subject suggest they might. Otten and her group plan to follow up in a year with that data as well.

But even with only so-called environmental changes to the restaurants, there was a marked improvement in their efforts to promote healthier options and provide more information on nutritional content. “We concluded that so far, the ordinance is positively influencing restaurants affected by the legislation,” says Otten.

That’s good news, since other cities are following suit. San Francisco passed similar legislation that will encourage fast-food chains to include more healthy options such as fruits and vegetables on their menus, and Otten is hopeful that the long-term effect of the ordinances will push restaurants to move beyond just changing their advertising practices to reformulating their menu options to help consumers eat healthier.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.